The next parts of the heats pumps are expansion valve and evaporator. The expansion valve is the pressure reducing devise.In heat pumps the evaporators is located outside the room and exposed to the atmosphere which is at very low temperature.
The expansion valve is the pressure reducing devise. When the high pressure, and medium temperature refrigerant enters the expansion valve its pressure reduces suddenly and along with it its temperature also becomes very low suddenly. The expansion valve most commonly used in the heat pumps is copper capillary tube. The refrigerant leaves the expansion valve at extremely low pressure and low temperature in partially liquid state and partially gaseous state.
While in the air conditioners the evaporator is located inside the room, in heat pumps the evaporator is located outside the room and exposed to the atmosphere which is at very low temperature. Just like condenser the evaporator is also made up of copper coil. The low pressure and low temperature refrigerant enters the evaporator coil, due to this the temperature of the coil also reduces drastically and it becomes even lower than the atmospheric temperature.
Since the temperature of the refrigerant inside the evaporator is less than the atmospheric temperature, it tends to absorb the heat from the atmosphere. The fan or the blower blows atmospheric air over the evaporator giving up the heat to the refrigerant and heating it. Since the refrigerant absorbs the heat from atmospheric air, its temperature increases, while its pressure remains constant and it get converted entirely into the gaseous state.
The low pressure and medium temperature refrigerant in gaseous state enters the compressor. In compressor the refrigerant is compressed to extremely high pressure and high temperature in gaseous state. This refrigerant then enters the condenser of the heat pump, where it heats the room air. In this the cycle of operation of the heat pump keeps on continuing. The refrigerant continuously flows through the closed cycle getting heated and cooled during each cycle. The refrigerant selected for the refrigerators and the heat pumps have the capacity to undergo phase changes without undergoing any chemical changes.